This invention relates to a liquid crystal light valve (LCLV) adapted to a large-scale image projector or an optical data processing apparatus, and more particularly an improved liquid crystal light valve capable of employing a titanium dioxide/silicon dioxide mirror without a bonding structure which enables the use of such a mirror.
There are, in general, two types of the liquid crystal light valve, one of which is a reflective type and the other of which is a transmissive type. Conventional LCLV utilizes electro-optical characteristics of the liquid crystal of which the dynamic property varies according to the quantity of the scanned input light, and modulates a projected output light. The LCLV is applied to amplify a light constituting an image or to vary a light wavelength. Such a LCLV is used in a light reflective and transmissive projecting system or an optical document processing apparatus.
In particular, the LCLV is employed in the design of infrared detectors for tracking missiles. Simulation of complex infrared radiation patterns from missiles has been performed. In the simulation, a LCLV for creating high resolution dynamic infrared images of a controlled or predetermined infrared radiation pattern and pattern history is employed. The LCLV of such an object is disclosed in U.S. Pat. No. 4,114,991 to Bleha, Jr. et al. The conventional reflective LCLV requires a very thick reflective film for operation in the 8-14 micron infrared region. Accordingly, in the above-mentioned U.S. Pat. No. 4,114,991, the incium-tin oxide reflective film is used to obtain a film of less than 0.3 microns thick. However, since a cadmium sulfide (CdS) photoconductor layer and a cadmium telluride (CdTe) light absorbing layer are used in the above U.S. Pat. No. 4,114,991, a response speed is slow and a manufacturing process is considerably more difficult.
Also, in order to operate the LCLV, the switching of a control voltage which dynamically controls a liquid crystal capable of double-refracting the output light by means of a photoconductor layer which is ionized by the input light having an image signal, should be followed. Thus, if there is no light, the impedance between the photoconductor layer and the reflective film should be maintained much greater than that of the liquid crystal. On the other hand, if there is light, the impedance should become much smaller than that of the liquid crystal. In such a LCLV, the impedance of the photoconductor layer should match with that of the liquid crystal because of the required operational property. A typical structure of the above-mentioned LCLV is disclosed in the U.S. Pat. No. 3,824,002 to Beard. The basic structure is the same as that shown in FIG. 1.
In FIG. 1, transparent electrodes 13 and 13a to which the AC driving voltage supplied from AC voltage source 10 is applied, are attached to the respective inner sides of two parallel transparent substrates 12 and 12a. Liquid crystal 19 located between transparent substrates 12 and 12a which maintain a specified distance by means of spacers 18 and 18a has two alignment films 17 and 17a attached to both sides thereof. The layers such as a photoconductor layer 14, a light absorbing layer 15 and a reflective film 16 are sandwitched between transparent electrode 13 formed on transparent substrate 12 where input light 1100 is incident, and an alignment film 17 is opposed to the transparent electrode 13. Unexplained reference numeral 11 in the drawing designates a coated layer which prevents input light from being reflected.
Also, in FIG. 1 the transparent substrate 12a facing toward output light 1200 receives infrared rays of light projected from the external source (not shown) and transmits the received infrared rays through the transparent electrode 13a, the liquid crystal 19 to reflective film 16. At this time, the projection light which is transmitted through the liquid crystal 19 is reflected by the reflective film 16 and returned back. The projection light 1200 of the infrared region is tuned to the variation of the polarized state which is induced according to the alignment variation of the liquid crystal molecules corresponding to the voltage variation across the liquid crystal due to the variation of the impedance which is induced by the photoconductor layer 14 according to the image of the visible input light 1100 incident in the opposite side.
In the above-mentioned LCLV, the photoconductor layer 14 is made of cadmium sulfide CdS, and the light absorbing layer 15 is made of cadmium telluride CdTe having an excellent matching characteristic with respect to the photoconductor layer 14. The LCLV having such a structure uses cadmium sulfide CdS having a low photosensitivity and a low response characteristic as a material of the photoconductor layer. For this reason, it is very difficult to create a dynamic image having high resolution and high frequency density.
To solve the above defect, a LCLV technology is disclosed in the U.S. Pat. No. 4,799,773. The LCLV disclosed in the U.S. Pat. No. 4,799,773 comprises an amorphous silicon (a-Si) photoconductor layer 24 and a cadmium telluride CdTe light absorbing layer 25, as shown in FIG. 2. In addition, this LCLV features an intermediate bonding layer 210 for properly contacting the above photoconductor layer 24 and the above light absorbing layer 25 to each other, that is, for compensating the different bonding properties between them.
Bonding layer 210 comprises four layers as shown in FIG. 3, that is, a first silicon dioxide (SiO.sub.2) layer 210a, a second oxygen-rich SiO.sub.2 layer 210b, a first oxygen-rich CdTe layer 210c and a second CdTe layer 210d.
However, since CdTe does not have a good mechanical bonding property with respect to the amorphous silicon (a-Si) the above LCLV must comprise a particular bonding layer 210. For this reason, a distinct manufacturing process is required, thereby raising the manufacturing cost.
As material of the light absorbing layer 25 in order to solve the above defects, a-SiGe, a-Si:F:H and so on are known. These materials however have defects in that the product is polluted and the film is damaged due to the elements of Ge, F etc. In fact, it is difficult to control due to the physical condition required as a light absorbing layer.